1. Field of the Invention
The present invention relates to an image detector including a capacitor for accumulating electric charges in an amount corresponding to an amount of an electromagnetic wave irradiated for recording. The present invention also relates to a method and an apparatus for recording image information on the capacitor as an electrostatic latent image by use of the detector and for retrieving the recorded electrostatic latent image.
2. Description of the Related Art
Facsimiles, copiers, radiation imaging apparatuses and the like, for example, are conventionally known as apparatuses using image detectors.
For example, in a radiation image recording apparatus for medical use or the like, methods are known for reducing a radiation dosage exposed to a subject and for improving diagnostic performance. According to such methods, a solid state radiation detector (an electrostatic recording medium) including a photoconductor (layer) such as a selenium plate, which is sensitive to radiation such as X-rays, is used as an image detector. The X-rays are irradiated onto the solid state radiation detector, and electric charges in an amount corresponding to a dosage of the irradiated radiation are accumulated in a capacitor inside the solid state radiation detector as electric charges for a latent image. In this way, radiation image information is recorded on the capacitor as an electrostatic latent image. Meanwhile, the solid state radiation detector recording the radiation image information is scanned with a laser beam or a line light source, whereby the radiation image information is retrieved out of the solid state radiation detector (as disclosed in Japanese Unexamined Patent Publication No. 6 (1994)-217322, U.S. Pat. No. 4,857,723 and Japanese Unexamined Patent Publication No. 9(1997)-5906, for example).
The method disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 6(1994)-217322 uses a solid state radiation detector formed by stacking a conductor layer, an X-ray photoconductive layer, a dielectric layer, and an electrode layer including multiple microplates corresponding to pixels, in which a charge retrieving thin-film transistor (TFT) is connected to each of the microplates. X-rays which are transmitted through an imaging object are irradiated onto the solid state radiation detector, so that electric charges for a latent image are accumulated in a capacitor formed between the microplates and the conductor layer. Accordingly, radiation image information is recorded on the solid state radiation detector. Thereafter, the electric charges for a latent image accumulated in the capacitor are read out to an outside of the solid state radiation detector by scan-driving the TFT. In this way, the radiation image information is retrieved.
Meanwhile, the method disclosed in the above-mentioned U.S. Pat. No. 4,857,723 uses a solid state radiation detector having a constitution of sandwiching both sides of a photoconductive layer with insulating layers, and further sandwiching the above-described object with stripe electrodes which include multiple line electrodes orthogonalized between the stripe electrodes on one side and the other side. X-rays which are transmitted through an imaging object are irradiated onto the solid state radiation detector, so that electric charges for latent images of mutually different polarities are accumulated in two capacitors, which are severally formed on interfaces between the photoconductive layer and the insulating layers in intersecting positions of the stripe electrodes on the both sides. Accordingly, radiation image information is recorded on the solid state radiation detector. Thereafter, the solid state radiation detector is scanned with a laser beam as retrieving light, whereby the electric charges for latent images accumulated in the capacitors are read out to an outside of the solid state radiation detector. In this way, the radiation image information is retrieved.
Meanwhile, the method disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 9(1997)-5906 uses a solid state radiation detector formed by stacking a first electrode layer, a recording photoconductive layer, a trap layer as a capacitor, a retrieving photoconductive layer and a second electrode layer in accordance with the order listed above. Uniform exposure light is irradiated in the state where high voltages are applied to electrodes provided on both sides in advance, thereby forming primary electrification at a capacitor of the detector. Thereafter, the both electrodes are either short-circuited, subjected to application of high voltages, or set open. In the state where electric fields are thus caused on the recording photoconductive layer, X-rays which are transmitted through an imaging object are irradiated onto the solid state radiation detector, so that electric charges for a latent image are accumulated in a capacitor. Accordingly, radiation image information is recorded on the solid state radiation detector. Thereafter, the both electrodes are short-circuited and then the solid state radiation detector is scanned with a laser beam as retrieving light, whereby the electric charges for a latent image accumulated in the capacitor are read out to an outside of the solid state radiation detector. In this way, the radiation image information is retrieved.
Furthermore, there has been disclosed a method of recording and retrieving a radiation image, which does not require provision of the charge retrieving TFT inside the electrode layer including the microplates unlike the method disclosed in the Japanese Unexamined Patent Publication No. 6(1994)-217322. Moreover, this method does not require a very expensive laser scanning system of a complex structure for reading out the electric charges of latent images accumulated inside the detector unlike the method disclosed in the U.S. Pat. No. 4,857,723. Further, this method does not require a light source for the primary electrification for forming the primary electrification at the capacitor of the detector unlike the method disclosed in Japanese Unexamined Patent Publication No. 9(1997)-5906. The method of recording and retrieving a radiation image includes the steps of: using a radiation image detector by stacking a first stripe electrode having multiple line electrodes, a photoconductive layer which takes on conductivity by receiving irradiation of an electromagnetic wave for recording, a capacitor, a rectifying layer formed with multiple diodes, and a second stripe electrode including multiple line electrodes formed as almost orthogonal to the first stripe electrode in accordance with the order listed above; applying a voltage between the both stripe electrodes; accumulating uniform electric charges on the capacitor; irradiating the electromagnetic wave for recording from the first stripe electrode side; and recording a radiation image by erasing the electric charges for a latent image on the capacitor in accordance with an amount of the electromagnetic wave. The method further includes the steps of: applying a given voltage severally between each line electrode of the first stripe electrode and the respective line electrodes of the second stripe electrode; and detecting a charged current flowing into the radiation image detector owing to application of the voltage with a current detection circuit connected between the respective line electrodes of the first stripe electrode and the second stripe electrode for performing retrieval.
However, the above-described radiation image recording and retrieving method performs recording by accumulating the electric charges for a latent image corresponding to the radiation image information into the capacitor, and the electric charges for a latent image also discharges electricity prior to retrieval. For example, a high-resistance (about 1014 to 1015Ω) photoconductive layer is formed if the photoconductive layer is made of a member mainly composed of a-Se. Accordingly, if the electric charges for a latent image accumulated in the capacitor are discharged from the photoconductive layer, such an amount of discharge is insignificant. On the contrary, if the photoconductive layer is made of a member mainly composed of a-Si, then it is difficult to form the photoconductive layer highly resistant as in the case of a-Se, whereby the photoconductive layer possesses relatively low resistance (about 104 to 1012Ω). Accordingly, an amount of discharge of the electric charges for a latent image from the capacitor cannot be ignored, and image quality such as an S/N ratio or sharpness is deteriorated upon image retrieval.